Friday, February 03, 2006

Early Animal Life Linked To Clay Production

Abstract: An enigmatic stepwise increase in oxygen in the late Precambrian is widely considered a prerequisite for the expansion of animal life. Accumulation of oxygen requires organic matter burial in sediments, which is largely controlled by the sheltering or preservation by detrital clay minerals in modern marine continental margin depocenters. Here we show mineralogical and geochemical evidence for an increase in clay mineral deposition in the Neoproterozoic that immediately predated the first metazoans. Today most clay minerals originate in biologically active soils, so initial expansion of a primitive land biota would greatly enhance production of pedogenic clay minerals (the "clay mineral factory"), leading to increased marine burial of organic carbon via mineral surface preservation.

Clay made animal life possible on Earth. A sudden increase in oxygen in the Earth's recent geological history, widely considered necessary for the expansion of animal life, occurred just as the rate of clay formation on the Earth's surface also increased, the researchers report.

"Our study shows for the first time that the initial soils covering the terrestrial surface of Earth increased the production of clay minerals and provided the critical geochemical processes necessary to oxygenate the atmosphere and support multicellular animal life,"

Analyzing old sedimentary rocks, the researchers found evidence of an increase in clay mineral deposition in the oceans during a 200 million year period that fell between 1.1 to 0.54 billion years ago – a stretch of time known as the late Precambrian when oxygen suddenly increased in the Earth's atmosphere. The increases in clay formation and oxygen shortly preceded – in geological time – the first animal fossils about 600 million years ago.

The study emphasizes the possibility that colonization of the land surface by a primitive terrestrial ecosystem (possibly involving fungi) accelerated clay formation, as happens in modern soils. Upon being washed down to the sea, the clay minerals were responsible for preserving more organic matter in marine sediments than had been the case in the absence of clays. Organic matter preservation results in an equal portion of oxygen released to the atmosphere through the chemical reaction of photosynthesis. Thus an increase in the burial of organic carbon made it possible for more oxygen to escape into the atmosphere.